We asked Michael Isikoff of Yahoo News, co-author of the new bestseller, “Russian Roulette: The Inside Story of Putin’s War on America and the Election of Donald Trump.” And the answer may be pretty simple: It’s all about business.“Everybody keeps looking for the smoking gun, the foolproof evidence that explains Trump’s Russia problems,” Isikoff tells Yahoo Finance. “But much of the evidence is hiding in plain sight.”

As a real-estate developer and head of the Trump Organization, Trump tried for years to build a Trump Tower in Moscow, just as he had in New York, Chicago, Vancouver, Istanbul and other marquis cities. By 2014, he was actually getting close. Trump had made the right connections during a 2013 trip to Moscow, where the Miss Universe pageant, which he owned, was held. He knew Putin’s personal approval would be necessary to build his tower. And by early 2014, his company had signed a letter of intent with Moscow authorities to go ahead with the project.

Trump’s daughter Ivanka, who was then one of the company’s top executives, was actually in Russia scouting for sites in February of that year, when Russia, under Putin’s order, annexed Crimea. Then Russian troops invaded Ukraine. Europe and the United States responded by imposing tough new economic sanctions on Russia—which directly affected a Russian bank Trump had lined up to help finance his Moscow tower.

“It was the sanctions that killed his Russia deal,” Isikoff says. “It helps explain a lot.”

As a candidate, Trump criticized those sanctions, without ever mentioning that they directly affected his business dealings. And as president, he has been strangely solicitous of Putin, bucking most of the U.S. national-security establishment and the orthodoxy of his own Republican party. Trump, for instance, called Putin recently to congratulate him for winning an “election” widely viewed as completely rigged. Trump’s own national-security advisers had strongly advised him not to congratulate Putin.

Trump persisted in his efforts to get a Moscow deal, even after declaring his candidacy for president in 2015. Later that year, Trump lawyer Michael Cohen—now embroiled in the Stormy Daniels controversy—spearheaded another effort to build a Trump tower in Moscow.

“The public knew nothing about it,” Isikoff says. That project never got off the ground, either.

While pursuing the Moscow deals, however, Trump and his company—including family members—developed a number of Russian contacts who circled back once Trump was a presidential candidate. Those were the Russians who offered “dirt” on Hillary Clinton, or helped set up meetings on the matter. A few are known confidantes of Putin. “If you want to understand what happened at these meetings,” Isikoff says, “you have to understand these business relationships.”

Special counsel Robert Mueller is now investigating whether any dealings between Trump people and Putin people may have broken the law. Trump still has no tower in Moscow, and there’s no public evidence he has done anything illegal. But Trump does have a looming problem with Mueller, who may know far more than he has revealed. Trump’s business ambitions have cast a long shadow.

This is why Trump doesn't want any investigation into his business. He calls it crossing a red line. Nevertheless, it's against the law for a government official to trade favors of government action for private advantage. As Wiki says:

Wiki wrote:Political corruption is the use of powers by government officials for illegitimate private gain. An illegal act by an officeholder constitutes political corruption only if the act is directly related to their official duties, is done under color of law or involves trading in influence.

Forms of corruption vary, but include bribery, extortion, cronyism, nepotism, parochialism, patronage, influence peddling, graft, and embezzlement. Corruption may facilitate criminal enterprise such as drug trafficking, money laundering, and human trafficking, though is not restricted to these activities. Misuse of government power for other purposes, such as repression of political opponents and general police brutality, is also considered political corruption. Masiulis case is a typical example of political corruption.